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ARROW-11386: [Release] Fix post documents update script

* Keep docs/c_glib/index.html that is generated by Jekyll.
* Fix wrong pull request URL.

Closes #9324 from kou/release-post-docs

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>